The metamaterial structures have been researched for applications to microwave circuits and their components and antennas, flat-plate super-lenses, near field imaging having a resolving power smaller than the wavelength, optical devices and their components of a cloaking technique or the like. The left-handed metamaterial simultaneously has a negative effective permittivity and a negative effective permeability and enables the propagation of backward waves. The majority of the applications of microwave circuits or antennas are based on a one-dimensional or two-dimensional left-handed metamaterial structure. Lately, a combination of split ring resonators and thin wires, and anisotropic/isotropic left-handed structures that use transmission line networks and dielectric spheres are also proposed.
In the metamaterial structure, the right-handed system indicates an electromagnetic wave propagation state in which the electric field vector, the magnetic field vector and the wave number vector of electromagnetic waves constitute the right-handed system, and also indicates the propagation state of forward waves (forward traveling waves) such that the direction of the transmitted power of electromagnetic waves (direction of the group velocity) and the direction of the flow of the phase plane (direction of the phase velocity) are directed in an identical direction. This state is possible in media and structures in which the effective permittivity and the effective permeability both have positive values.
Moreover, in the metamaterial structure, the left-handed system indicates an electromagnetic wave propagation state in which the electric field vector, the magnetic field vector and the wave number vector have relations that constitute the left-handed system, and also indicates the propagation state of backward waves (backward traveling waves) such that the direction of the transmitted power of electromagnetic waves and the direction of the flow of the phase plane are directed in opposite directions. This state is possible in media and structures in which the effective permittivity and the effective permeability both have negative values.
Several constituting methods of metamaterials are proposed, and the two of resonant type metamaterials and transmission line (non-resonant) type metamaterials can be enumerated as representative examples.
The former resonant type metamaterials are configured of combinations of magnetic and electric resonators that respond to the magnetic-field and electric-field components of external electromagnetic fields as represented by a combination of split ring resonators configured to include metal strips and thin wires. This structure, in which the effective permittivity or the effective permeability exhibits an anti-resonance characteristic, therefore receives a very large influence from a loss in the vicinity of the resonant frequency.
On the other hand, the latter transmission line type metamaterial has its structure configured by using the fact that the general electromagnetic wave propagation form can be described by a transmission line model. In contrast to the fact that the conventional one-dimensional right-handed metamaterial structure that permits forward wave propagation takes a ladder type structure in which inductive elements are inserted in the series branch and the capacitive elements are inserted in the parallel branch (shunt branch), the one-dimensional left-handed metamaterial structure takes a structure in which the capacitive elements are inserted in the series branch and the inductive elements are inserted in the parallel branch in order to make the values of the effective permittivity and the effective permeability negative. The majority of the transmission line type metamaterials, which exhibit no anti-resonance characteristics in the effective permittivity and permeability, therefore have a feature that they have a lower loss than that of the resonance type. The transmission line type metamaterial, which operates as a right-handed metamaterial, a left-handed metamaterial, a single negative metamaterial such that either one of the permittivity and the permeability becomes negative and the other becomes positive or a metamaterial such that the effective permittivity or permeability is zero depending on the operating frequency band, are therefore called the composite right-handed/left-handed metamaterials.
The frequencies at which the effective permittivity and the effective permeability of the composite right-handed/left-handed metamaterial each having a value of zero are generally different from each other. In the above case, in a band between the frequency at which the adjacent effective permittivity is zero and the frequency at which the effective permeability is zero, the frequencies being adjacently located, either one of the effective permittivity and the effective permeability is negative, and the other has a positive value. In this case, the propagation condition of electromagnetic waves is not satisfied, and a forbidden band is formed. The metamaterial operates as a left-handed metamaterial in the band on the downside of this forbidden band since the effective permittivity and the effective permeability are both negative or operates as a right-handed metamaterial in the band on the upside where both of them have positive values. When the frequencies at which the effective permittivity and the effective permeability are zero coincide with each other, no forbidden band is formed, and the left-handed transmission band and the right-handed transmission band are continuously connected. Such a metamaterial is called the balanced composite right-handed/left-handed metamaterial, and the other is called the unbalanced composite right-handed/left-handed metamaterial. The balanced composite right-handed/left-handed metamaterial has the features that the forbidden band is not generated, and also the group velocity does not become zero even at the frequency at which the phase constant is zero, allowing efficient transfer of power to be achieved.